In LifeChanger, sharing is performed at the List level. See here for more details.  You can share individual tasks by sharing a List with certain tag(s) and making sure the task you want to share has that tag.   e.g. to share a task with Emma you could create a tag called Emma and assign that to the task(s) you want to share.  Then share the list with Emma ensuring that only tasks with the Emma tag are shared.